A silicon single crystal is manufactured by being pulled up and grown based on CZ (Czochralski method).
FIG. 1 shows a configuration example of a silicon single crystal manufacturing device 1.
A CZ furnace 2 is internally provided with a quartz crucible 3 for melting a polycrystalline silicon raw material and housing it as a melt 5.
Polycrystalline silicon (Si) is heated and melted in the quartz crucible 3. When the temperature of the melt 5 is stabilized, a pulling mechanism 4 is operated and the silicon single crystal 10 is pulled up from the melt 5.
During the pulling, the quartz crucible 3 is rotated with a rotational axis 15. The rotational axis 15 can be driven in the vertical direction, and is able to move the quartz crucible 3 vertically to an arbitrary crucible position, and thereby adjust a surface 5a of the melt 5, namely, the liquid level H of the melt 5.
Moreover, a thermal shield 8 (heat radiation plate, gas straightening tube) is provided above the melt 5 and around the silicon single crystal 10. A rim 8a is provided to the lower end of the thermal shield 8.
The thermal shield 8 guides, within the CZ furnace 2, argon gas 7 as the carrier gas to be supplied from the upper side to the center of the melt surface 5a, and further guides it to the peripheral edge of the melt surface 5a by passing it through the melt surface 5a. The argon gas 7 is thereafter discharged from an outlet provided to the lower part of the CZ furnace 2 together with the gas that evaporated from the melt 5. Consequently, it is possible to stabilize the gas flow rate on the liquid level and maintain the oxygen that evaporated from the melt 5 in a stable state.
Moreover, the thermal shield 8 insulates and shields a seed crystal 14 and the silicon single crystal 10 grown from the seed crystal 14 from the heat radiation generated in the hot areas such as the quartz crucible 3, the melt 5, and a heater 9. The thermal shield 8 also prevents impurities (for instance, silicon oxide) generated in the furnace from adhering to the silicon single crystal 10 and inhibiting the single crystal growth.
The size of the distance L between the rim 8a at the lower end of the thermal shield 8 and the melt surface 5a (hereinafter referred to as the “thermal shield/liquid level distance”) can be adjusted by lifting and lowering the rotational axis 15 and changing the vertical position of the quartz crucible 3. The distance L can also be adjusted by moving the thermal shield 8 in a vertical direction using a lifting and lowering device.
Quality of the silicon single crystal 10 is conventionally known to fluctuate in accordance with the size of the melt liquid level H or the thermal shield/liquid level distance L during the pulling. Specifically, if the size of the melt liquid level H or the thermal shield/liquid level distance L changes during the pulling, parameters such as the temperature gradient in the axial direction of the silicon single crystal 10 consequently fluctuate, thereby causing the defect region distribution and oxygen concentration distribution of the silicon single crystal 10 to change, whereby the quality of the crystal also change.
Thus, in order to obtain the crystal quality of the demanded specification, pulling conditions, that is, the target value of the melt liquid level H for each pulling position or the target value of the thermal shield/liquid level distance L for each pulling position is predetermined according to the demanded specification. During the pulling and growing process, the actual values of the melt liquid level H or the actual values of the thermal shield/liquid level distance L are sequentially detected, these detected values are fed back, and control is performed to adjust the vertical position of the rotational axis 15 so that the deviation of the target value and the detected value becomes zero.
Accordingly, in order to stably obtain a crystal quality of the demanded specification, the melt liquid level H or the thermal shield/liquid level distance L must be controlled to accurately coincide with the target value. In order to realize the above, the actual value of the melt liquid level H or the actual value of the thermal shield/liquid level distance L as the feedback amount to be detected during the control must constantly be measured with accuracy.
FIG. 2 shows a configuration example of the distance measuring device for measuring the actual value of the melt liquid level H or the thermal shield/liquid level distance L for each pulling position.
The distance measuring device 100 of FIG. 2 is configured by including light emitting means 110 for emitting a laser beam 101, light scanning means 120 for scanning the laser beam 101 that is emitted from the light emitting means 110 along the radial direction of the quartz crucible 3, light receiving means 130 for receiving the reflected light of the laser beam 101 that is emitted from the light emitting means 110 and used to perform scanning of the light scanning means 120, and pulling distance measuring means 141 for measuring the thermal shield/liquid level distance L or/and the melt liquid level H based on the fixed scanning position during the pulling, the laser beam emitting position of the light emitting means 110 and the light receiving position of the light receiving means 130, and according to the principle of triangulation.
The light scanning means 120 is configured by including a mirror 121 for reflecting the laser beam 101 that is emitted from the light emitting means 110, and a stepping motor 122 for changing the attitude angle of a light reflecting surface 121a of the mirror 121.
Here, the rotation angle θ of the rotational axis 122a of the stepping motor 122 and the scanning position of the laser beam 101 in the radial direction of the crucible 3 correspond one-to-one. Thus, in this specification, the scanning position of the laser beam 101 in the crucible radial direction is represented as θ.
Patent Document 1 discloses a method of measuring the actual value of the melt liquid level H or the actual value of the thermal shield/liquid level distance L as follows.
Specifically, foremost, the stepping motor 122 is driven to position the optical scanning position θ at the pulling position θ1. Subsequently, the laser beam 101 is emitted from the light emitting means 110 and irradiated onto the melt level 5a, and the laser beam that reflects off the melt level 5a is received by the light receiving means 130. Subsequently, the distance LS from the reference point to the liquid level 5a of the melt 5 is sought and the melt liquid level H is measured based on the fixed scanning position θ1 during the pulling, the emitting position of the light emitting means 110, and the light receiving position of the light receiving means 130, and according to the principle of triangulation.
Subsequently, the stepping motor 122 is driven to position the optical scanning position θ at the pulling position θ2. Subsequently, the laser beam 101 is emitted from the light emitting means 110 and irradiated onto the upper surface 8b of the rim 8a of the thermal shield 8, and the laser beam that reflects off the rim upper surface 8b is received by the light receiving means 130. Subsequently, the distance S from the reference point to the upper surface 8b of the rim 8a of the thermal shield 8 is measured based on the fixed scanning position θ2 during the pulling, the emitting position of the light emitting means 110, and the light receiving position of the light receiving means 130, and according to the principle of triangulation. The thermal shield/liquid level distance L is calculated based on the obtained distance LS up to the liquid level 5a of the melt 5, the distance S up to the upper surface 8b of the rim 8a of the thermal shield 8, and the thickness t of the rim 8a of the thermal shield 8.
Moreover, Patent Document 2 discloses a method of measuring the actual value of the melt liquid level H and the actual value of the thermal shield/liquid level distance L as follows.
Specifically, as shown in FIG. 3, foremost, the stepping motor 122 is driven to position the optical scanning position θ at the pulling position θ3. Subsequently, the laser beam 101 is emitted from the light emitting means 110 to be reflected off the melt level 5a, the laser beam that reflects off the melt level 5a is irradiated onto the lower surface 8c of the rim 8a of the thermal shield 8, the laser beam that reflects off the rim lower surface 8c is irradiated onto the melt level 5a once again, and the laser beam that reflects off the melt level 5a is received by the light receiving means 130. Subsequently, the distance LS from the reference point to the liquid level 5a of the melt 5 is sought and the melt liquid level H is measured based on the fixed scanning position θ3 during the pulling, the emitting position of the light emitting means 110, and the light receiving position of the light receiving means 130, and according to the principle of triangulation.
Subsequently, the stepping motor 122 is driven to position the optical scanning position θ at the pulling position θ4. Subsequently, the laser beam 101 is emitted from the light emitting means 110 and irradiated onto the upper surface 8b of the rim 8a of the thermal shield 8, and the laser beam that reflects off the rim upper surface 8b is received by the light receiving means 130. Subsequently, the distance S from the reference point to the upper surface 8b of the rim 8a of the thermal shield 8 is measured based on the fixed scanning position θ4 during the pulling, the emitting position of the light emitting means 110, and the light receiving position of the light receiving means 130, and according to the principle of triangulation. The thermal shield/liquid level distance L is calculated based on the thus obtained distance LS up to the liquid level 5a of the melt 5, the distance S up to the upper surface 8b of the rim 8a of the thermal shield 8, and the thickness t of the rim 8a of the thermal shield 8.
The foregoing optical scanning positions θ1, θ2, θ3, θ4 during the pulling are defined based on the reference optical scanning position θc. The reference optical scanning position θc is the edge 8e of the rim 8a of the thermal shield 8.
Patent Document 2 discloses the position measuring algorithm for measuring the optical scanning position θc of the edge 8e of the rim 8a of the thermal shield 8. This position measuring principle is explained with reference to FIG. 4. This position measuring algorithm is performed, for instance, between the respective batches, during the disassembly or cleaning of the CZ furnace 2, or midway during the pulling process.
Specifically, foremost, the distance between the reference point and the reflection point is sequentially measured for each prescribed interval Δθ1 based on the sequential optical scanning position, the emitting position of the light emitting means 110, and the light receiving position of the light receiving means 130, and according to the principle of triangulation while scanning the laser beam 101 by the light scanning means 120 in the radial direction of the crucible 3.
Subsequently, it is determined whether the measured distance changes from a size corresponding to the distance between the reference point and the melt 5 to a size corresponding to the distance between the reference point and the rim 8a of the thermal shield 8.
If it is consequently determined that the measured distance changes, it is determined that the laser beam 101 is reflected off the edge 8e of the rim 8a of the thermal shield 8 at the optical scanning position θc at the point in time that the change is determined.
As described above, the positions θ1, θ2, θ3, θ4 of the direction for scanning the laser beam 101 during the pulling are defined as reference based on the optical scanning position θc when it is determined that the laser beam 101 reflected off the edge 8e of the rim 8a of the thermal shield 8.    Patent Document 1: Japanese Patent Application Laid-Open No. 2000-264779    Patent Document 2: WO01/083859